Preparation, acid modification and catalytic activity of kaolinite nanotubes in α-pinene oxide isomerization

In this work kaolinite nanotubes (KNT) were obtained from commercial kaolin AKF-78 (Uzbekistan) by starting material sequential intercalation by DMSO and methanol, followed by treatment with a cetyltrimethylammonium chloride solution. Acid functionalization of KNT for catalytic applications was successfully performed for the first time using a two-step treatment with piranha solution (H2SO4–H2O2), which resulted in the removal of organic impurities as synthetic artifacts and an increase in specific surface area by 3.9 times (up to 159 m2 g−1), pore volume by 1.5 times (0.23 cm3 g−1) and acidity by 4.1 times (49 μmol g−1). The values of the porous structure parameters and concentration of acid sites in processed kaolinite nanotubes practically corresponded to those for natural halloysite nanotubes (HNT) modified in the same way. Both types of materials demonstrated catalytic activity in the model reaction of α-pinene oxide isomerization in various solvents, including green ones, with selectivity to trans-carveol up to 55–57% and campholenic aldehyde of 50–51%, depending on the medium used. A satisfactory correlation between solvent polarity and selectivity was also observed. To the best of our knowledge, this is the first example of using modified kaolinite nanotubes per se as a catalyst. Overall, treatment of KNT with piranha solution provides not only catalytic activity but also the opportunity for further functionalization and application of these nanomaterials.


Introduction
A representative of natural materials with a nanotubular structure is halloysite, a mineral of the kaolinite group, the layer of which consists of tetrahedral Si-O and octahedral Al-O networks. 12][3] For example, acid-modied halloysite is an effective catalyst for the Prins reaction of unsaturated compounds with aldehydes for the synthesis of products with benzopyran, 4 tetrahydropyran, 5 isobenzofuran 6 and other types of structures. 7There are examples of the use of functionalized HNT in the Biginelli reaction for synthesis of 3,4-dihydropyrimidinones, 8 preparation of 5-ethoxymethylfurfural as a biofuel component, 9 and in a number of other transformations. 2,3n recent years, as an alternative to natural halloysite, synthetic nanotubes have been studied, which can be obtained from kaolinite by sequential introduction (intercalation) of reagents into its interlayer space with subsequent directed deformation of the layers by chemical and ultrasonic treatment, [10][11][12][13][14][15] which can be illustrated in Fig. 1.
][12] Intercalation of Chinese kaolinite with DMSO, MeOH followed by exposure to a methanol solution of cetyltrimethylammonium chloride (CTACl) for 24 h at temperatures from 30 to 100 °C led to formation of kaolinite nanotubes (KNT). 10KNT was prepared by stirring kaolin clay intercalated with methanol in a CTACl solution at room temperature (72 h) followed by ultrasound treatment. 11reatment with methanol is one of the problematic stages in the synthesis of nanotubes, since the duration of the treatment is 7-10 days 10,11 with the amount of reagent reaching up to 200 mL g −1 due to need to replace it ten times with the fresh solvent.Reducing the time of such intercalations can be achieved by using a Soxhlet extractor avoiding the need for periodic replacement of methanol, thereby reducing the solvent consumption to 20 mL g −1 . 14ecently, the authors developed a method for producing nanotubes from natural kaolin (Dedovka mine, Belarus) with a length of 800-1100 nm and a diameter of 50-60 nm. 15 The synthesis was carried out under relatively mild conditions (60-66 °C, atmospheric pressure) and low (7.0 mL g −1 ) amounts of MeOH required for their formation.
Acid functionalization of natural halloysite nanotubes can be performed by graing -SO 3 H groups on their surface, 2,3,9 or by treatment with acid solutions, [2][3][4][5][6][7][8]16 including HCl, 4-7 H 3 PO 4 , 16 and a mixture of H 2 SO 4 -H 2 O 2 (piranha solution), 8 leading to an increase in both Lewis and Brønsted acidity.[4][5][6][7][8]16 Although acidic halloysite nanotubes are active, selective, and stable catalysts for a number of organic reactions, 2-10 to the best of our knowledge, the catalytic of the properties the materials based on their synthetic kaolinite analogues have not been reported. Mor2][3] It can be expected that their synthetic analogues will have more constant composition and properties, which makes them promising materials for further functionalization.
][19] For example, selective formation of campholenic aldehyde (up to 96%) was observed on Ti-MCM-22 containing Lewis acid sites (tetrahedrally coordinated Ti species). 22The same high yield of this product (95%) was observed in the presence of Mo IV complexes in ionic liquids. 23Quite high selectivity to campholenic aldehyde (82%) can also be achieved on Cu/MCM-41 at 70 °C in toluene. 24The mechanism of campholenic aldehyde formation in the presence of Fe 3+ ions (up to 65%) was also studied by DFT computation in ref. 25.On layered aluminosilicates with weak acidity, the formation of almost equal amounts of campholenic and iso-campholenic aldehydes (68% in total) was observed. 21][19] Thus, high selectivity to trans-carveol (up to 93%) was observed in the presence of heteropoly acid H 3 PW 12 O 40 in N,Ndimethylformamide. 26 Carbon materials functionalized with acid groups, in particular biochar 27 and microspheres, 28 have shown high efficiency in the selective isomerization of a-pinene epoxide to carveol (up to 85%) in the same solvent.In ref. 29  phosphorus anion-based ionic liquids having alkyl ammonium, pyridinium or lutidinium cations were utilized for synthesis of trans-carveol with 74% selectivity at 99% substrate conversion.In the presence of hierarchical beta zeolites, a carveol yield of up to 42% was observed in N,N-dimethylacetamide at 140 °C on materials with well-developed mesoporosity and a high fraction of Brønsted acid sites of weak and medium strength. 30he purpose of this work is to (i) develop efficient method for the acid functionalization of kaolinite nanotubes (KNT), and (ii) compare their textural, acidic and catalytic properties (in isomerization of a-pinene oxide) with those of modied natural halloysite nanotubes.

Synthesis and characterization of materials
Uzbekistan kaolins from Sulton Uvays, Alyans and Angren (grades AKF-78 and AKS-30) mines were used as raw materials.The main attention was paid to commercial kaolin AKF-78 due to its availability.To clean the surface from inorganic impurities, which facilitates further intercalation, 31 kaolin samples were washed with 5% HCl.In a three-neck ask 30 g of clay and 150 mL of an acid solution were placed, heated to 80 °C and stirred at this temperature for 1 h.The precipitate was separated and washed with distilled water on a lter until the absence of Cl − , dried for 3 h at 105 °C and ground into powder.Halloysite nanotubes (HNT) were purchased from Sigma-Aldrich (origin Dragon Mine, USA).
The following typical procedure was used to synthesize aluminosilicate nanotubes. 15To a three-neck ask equipped with a mechanical stirrer, a thermometer, a reux condenser, 10 g of kaolin, 60 mL of DMSO and 10 mL of distilled water were added.The contents were heated to 90 °C and stirred at this temperature for 6 h.Then the solid phase was separated on a lter and dried for 3 h at 60 °C, resulting in the DMSO form of kaolinite.
A sample of the resulting powder (3 g) in a lter paper sleeve was placed in a Soxhlet apparatus, into the cube of which Fig. 2 A general scheme of the catalytic isomerization of a-pinene epoxide (1) into campholenic aldehyde (2), iso-campholenic aldehyde (3), pinocarveol (4), isopinocamphon (5), trans-carveol (6), trans-sorberol (7), p-cymene (8) and pinol (9)  The MeOH-kaolinite was transferred to a three-neck ask, 100 mL of a methanol solution of CTACl (concentration 0.1 or 1.0 mol L −1 ) was added, heated to boiling (66 °C) and stirred at this temperature for 24 h using a magnetic stirrer.The solid phase was then separated, washed four times with hot ethanol, dried at 150 °C for 3.0 h.As a result, a material containing kaolinite nanotubes (KNT) was obtained.
Acid functionalization of kaolin nanotubes was carried out with a piranha solution (a mixture of 95% H 2 SO 4 and 30% H 2 O 2 , volume ratio 3 : 1), choice of which was due to the need for (i) removal of organic impurities from them and (ii) acid functionalization.A sample of KNT (1.0 g) was placed in a three-neck ask, 5.0 mL of piranha solution was added and slowly heated to 40-50 °C, when gas evolution began.Aer destruction of the impurities, the mixture was heated to 90 °C and stirred at this temperature for 1 h.Then the solid phase was ltered, thoroughly washed with distilled water until acids and hydrogen peroxide were absent from the washing, dried (150 °C, 3.0 h) and fractionated (100 mm).This solid was designated as KNT-Pir.
The treatment of halloysite nanotubes was carried out according to a similar procedure, but only at 90 °C for 1 h due to the absence of organic impurities.
The porous structure of solids was measured with nitrogen physisorption on ASAP 2020 MP (Micromeritics) analyzer.The samples (ca.50 mg) were previously evacuated (residual pressure 0.013 Pa) for 1 h at 200 °C.The specic surface area was calculated by the Brunauer-Emmett-Teller equation. 32The volume and the average diameter of the pores were determined from the desorption branch of the isotherm using the Barrett-Joyner-Halenda method. 33The chemical composition of the samples was determined by energy dispersive X-ray spectroscopy (EDX) using an electron microscope with a JEOL JCM-6000Plus chemical analysis system (Japan).
FTIR spectra of the samples were recorded in the region of 4000-400 cm −1 on a Bruker Tensor 27 spectrometer.Images solids obtained from kaolin were recorded using (i) a Zeiss Leo 1530 scanning electron microscope, as well as (ii) a LEO 906E transmission electron microscope.For the last one the samples were preliminarily dispersed in a water-ethanol mixture (3 : 1), applied to grids and secured by sputtering a carbon lm.
The acidity of the catalysts was measured by infrared spectroscopy (Shimadzu, IRTracer-100) using pyridine as a probe molecule.A thin self-supported wafer of the catalyst (10-30 mg) was pressed and then placed into the FTIR cell.The catalyst wafer was outgassed at 350 °C for 1 h.Thereaer, the temperature was decreased to 100 °C and background spectra was recorded.Pyridine was adsorbed on the sample for 30 min at 100 °C followed by desorption at 150, 250 and 350 °C for 1 h and the spectra of the sample were recorded in between every temperature ramp.The scanning was performed under vacuum at 100 °C.Spectral bands at 1545 and 1450 cm −1 were used to identify Brønsted and Lewis acid sites, respectively.The quantitative amount of the acid sites was calculated using the extinction coefficient of Emeis. 34ote that the physicochemical properties of the parent and acid-treated HNT were carefully studied by XRD, EDX, 27 Al and 29 Si MAS NMR, SEM, TEM, FTIR with pyridine, and N 2 adsorption and thermal analysis methods. 4

Catalytic tests
To carry out isomerization of a-pinene oxide, 0.5 g of this compound was added to a two-neck ask (50 mL) equipped with a thermal controller and a magnetic stirrer, then 10 mL of the solvent was added.Aer heating the contents to the required temperature, 25 mg (5.0 wt%) of solid was added there and stirring was carried out (300 rpm), periodically taking samples for analysis.The composition of the reaction mixture was determined by gas-liquid chromatography using a Khromos GKh-1000 equipment with a ame ionization detector and a Zebron Zb-5 capillary column according to the method described in ref. 21.

Physicochemical characteristics of KNT and comparison with HNT
The diffraction patterns of kaolin clays washed with 5% HCl (Fig. 3) show reections characteristic of kaolinite with interplanar distances of about 7.0 and 3.6 Å. 10,11 Peaks around 10.0 and 3.3 Å indicate the presence of illite and quartz as impurities in all samples. 21The highest reection intensity at 7 Å is characterized by Alyans and AKF-78 kaolins (Fig. 3).
The synthetic procedure was initially implemented using a 0.1 mol per L CTACl solution in methanol.Scanning electron microscopy (SEM) images of Sulton Uvays and Alyans kaolins (Fig. 4) obtained aer such treatment contain plate-like particles characteristic of kaolinite, and in the case of the rst sample, the particles with tubular morphology are also observed (Fig. 4a).Treatment of commercial AKC-30 and AKF-78 under similar conditions also resulted in the formation of nanotubes, with the largest number of nanotubes found in the AKF-78 sample (Fig. 5b).However, according to SEM, unconverted kaolin predominates in all samples (Fig. 4 and 5).
Further synthesis of KNT was carried out on the basis of commercial clay AKF-78, using a more concentrated CTACl solution (1.0 mol L −1 ), similar to the procedure described in ref. 15.According to the transmission electron microscopy image, under these conditions, nanosized tubes with a length of 600-1000 nm and a diameter of 15-25 nm were formed (Fig. 6b).At the same time, particles of the parent kaolin were also observed in small quantity.
The FTIR spectrum of the parent kaolinite AKF-78 (Fig. 7) shows intense absorption in the region of 1200-1000 cm −1 with peaks at 1109, 1033 and 1009 cm −1 , which correspond to stretching vibrations of Si-O in its structure. 10,11,15A set of bands in the region of 600-400 cm −1 is associated with bending Si-O vibrations, among which the lines at 538 and 470 cm −1 belong to the Si-O-Al and Si-O-Si fragments, respectively. 10,11,15bsorption at 914 cm −1 is due to bending vibrations of Al-OH groups. 10,11,15 the region of 3700-3500 cm −1 , the lines are observed at 3620 and 3697 cm −1 , which are caused by stretching vibrations of structural -OH groups (Fig. 7).The last band is associated with vibrations of hydroxyls, near the surface of the octahedral (Al-O) layers of kaolinite and capable of forming hydrogen bonds with the tetrahedral (Si-O) layers. 10,11,15A broad  The FTIR spectrum of nanotubes obtained from AKF-78 is almost identical to that of the starting material (Fig. 7), indicating that the elements of the kaolinite structure are preserved.The doublet that appears at 2924 and 2852 cm −1 (-CH 2and CH 3 -) clearly indicates the presence of organic compounds in the resulting nanotubes. 10,11,15The broad line in the region of 3434 cm −1 , observed for AKF-78, sharply decreases in intensity and shis to 3534 cm −1 , which indicates a signicant decrease in the content of adsorbed water in KNT, apparently also due to organic impurities.Such impurities are typical for kaolinite nanotubes and remain as a result of incomplete removal of reagents aer the synthesis procedure. 10,11,15s a result of the kaolinite nanotubes treatment with a piranha solution, a signicant decrease in the intensity of the peaks around 2923 and 2853 cm −1 occurs in their FTIR spectrum, while absorption in the 3600-3400 cm −1 region (3430 cm −1 , H 2 O) sharply increases in intensity, becoming close to that of the original kaolin AKF-78 (Fig. 7).Based on this, it can be assumed that such treatment leads to the removal of a signicant number of organic impurities and an increase in the hydrophilicity of the nanotube surface.Noted that the shape and intensity of the lines at 538, 1034 (Si-O), 3621 and 3699 cm −1 (-OH) do not change, which clearly indicates the preservation of the crystalline structure of the nanotubes aer exposure to the H 2 SO 4 -H 2 O 2 mixture.This is also clearly conrmed by the TEM image of KNT-Pir (Fig. 6c), according to which the shape and size of the nanotubes did not change.
The composition of the parent kaolinite AKF-78 and halloysite includes mainly Al 2 O 3 and SiO 2 , as well as small amounts of oxides of iron, sodium, magnesium, potassium, calcium, and titanium (Table 1).A higher SiO 2 content in AKF-78 originated from the presence of quartz as an impurity, as evidenced by XDR data (Fig. 3).The commercial halloysite used in this work practically does not contain any impurities. 4Note that in the process of obtaining nanotubes from kaolinite, a change in the composition of inorganic substances should not occur, since the synthetic procedure is aimed at changing the morphology of its particles, 10,11,15 while organic impurities are present in the resulting KNT (Fig. 7).
Treatment of both KNT and HNT with the piranha solution leads to a decrease in Al 2 O 3 content and an increase in the amount of SiO 2 in the resulting samples (Table 1).This is because of the mechanism of layered aluminosilicates acid etching, which involves the breaking of Si-O-Al bonds between their tetrahedral silicon-oxygen and octahedral aluminumoxygen layers, ultimately leading to leaching of Al 3+ into the solution and formation of amorphous SiO 2 . 4,20,35he same as for KNT (Fig. 6b and c), treatment of HNT with a piranha solution does not lead to the destruction of nanotubes, since their shape and size (Fig. 8) correspond to those for Dragon Mine type halloysite. 4,36According to ref. 4, signicant degradation of halloysite nanotubes occurs when more than 50% of the Si-O-Al bonds are destroyed.
The specic surface area (S BET ) of the original AKF-78 kaolin equal 33 m 2 g −1 was slightly increases for the nanotubes obtained from it (41 m 2 g −1 , Table 1).Note that an almost similar S BET value (39 m 2 g −1 ) was observed for KNT synthesized on the basis of commercial (Sigma-Aldrich) kaolin. 14For the parent halloysite, the specic surface area is also comparable (60 m 2 g −1 , Table 1).
Pretreatment of KNT and HNT with the piranha solution resulted in an increase in both S BET and pore volume.At the same time, the specic surface area values for the acid-modied KNT/HNT-Pir materials are almost the same (159-165 m 2 g −1 ).Activation of halloysite from China (Hubei) using a similar method led to an increase in S BET to 73 m 2 g −1 . 37On the other hand, a higher surface area of HNT (up to 207 m 2 g −1 ) was observed aer their treatment with 30% HCl at 90 °C, which, however, caused almost complete destruction of the tubular structure. 4ecently, 38 using the method of molecular dynamic modeling, it has been shown that during Al leaching, some nanoporous (1-20 nm) interlayer spaces are formed inside the wall of HNT due to the strong bonds between the adjacent layers.The interlayer nanopores greatly increase the specic surface area and the pore volume of halloysite, which is more prominent aer 50% Al leaching.These conclusions correspond to the results obtained earlier in ref. 4 which during halloysite HCl treatment the highest values of S BET and V pore were aer 50% removal of aluminum.
Worth noting that the activation of KNT with an HCl solution is ineffective, since in this case the formation of a gel was observed, apparently due to the interactions of the acid with the CTACl impurity, and the resulting material did not have any catalytic activity.Isotherms of low-temperature nitrogen adsorption-desorption for solids based on kaolin AKF-78 and halloysite are presented in Fig. 9.All of them belong to type IV(a), according to the IUPAC classication. 39Narrow capillary-condensation hysteresis loops on the isotherms belong to type H3 and have two distinctive features: (i) their adsorption branches resemble type II isotherms, and (ii) the lower boundary of the desorption branches is at the level of the relative cavitation pressure, p/p 0 ∼ 0.42.Loops of the type H3 are formed, as a rule, by non-rigid aggregates of lamellar particles (usually in layered minerals).Thus, the obtained isotherms are typical for mesoporous materials; the average pore diameter calculated from their desorption branch is 10.3-15.7 nm (Table 1).
The concentration of acid sites (a.s.) on the AKF-78 kaolin surface is 15.0 mmol g −1 , decreasing to 12.0 mmol g −1 for the nanotubes obtained from it (Table 2).The lower acidity value may be due to blocking of the active sites by organic impurities that are present in KNT (Fig. 7).Treatment of kaolinite nanotubes with a piranha solution leads to a sharp increase in the number of acid sites (49.0 mmol g −1 ), primarily Lewis ones.It is known that during the acid etching of layered aluminosilicates (including kaolinite, halloysite, montmorillonite, etc.), a cation exchange occurs leading to the formation of the H + form, which is unstable and is converted spontaneously to the Al 3+ form by the release of structural Al 3+ cations onto the particles surface. 35,40An increase in the L/B ratio was also observed when illite clay was treated with HCl, while the total concentration of a.s. in the resulting catalyst (up to 47.0 mmol g −1 (ref.21)) was the same as that for KNT-Pir (49.0 mmol g −1 ).
The amount of a.s.for the starting halloysite nanotubes is 34.0 mmol g −1 , being signicantly lower than for kaolin (15.0 mmol g −1 , Table 2).Note that due to the almost complete absence of isomorphism in kaolinite and halloysite, the active sites should be located at the ends and faces of their particles. 4,35The higher dispersion of halloysite particles is obviously the reason for its larger acidity.Treatment of HNT with the piranha solution also leads to an increase in the number of a.s. up to 59.0 mmol g −1 , which is slightly higher than in the case of KNT-Pir (49.0 mmol g −1 ).At the same time, the Brønsted type of acidity predominates in HNT (Table 2).
According to ref. 8, etching halloysite (Sigma-Aldrich) with the piranha solution gives the same acidity value (59.8 mmol g −1 with L/B 1.6, measured at 200 °C).The reasons for the low concentration of Brønsted a.s. in KNT-Pir compared to HNT processed under similar conditions require additional detailed studies.
Note that the treatment of HNT (Sigma-Aldrich) with HCl allows one to increase its acidity up to 52.0 mmol g −1 , 4 whereas in the case of H 3 PO 4 its values are equal to 45.0 mmol g −1 . 16In addition, effective methods for acid-functionalizing silicates, including halloysite nanotubes, is the xation of -SO 3 H groups on their surface using various reagents (chlorosulfonic acid, 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane, ionic liquids, etc.) under elevated temperatures 3,9,41,42 or an electron beam irradiation. 43Based on this, graing of various functional groups on the surface of KNT aer cleaning from organic impurities by the piranha solution is a promising direction for further work.

Catalytic activity of acid-functionalized HNT and KNT
For an initial screening of a-pinene oxide isomerization, halloysite treated by the piranha solution was used as a more accessible catalyst.][19] In the presence of non-polar solvents (cyclohexane, p-cymene), the isomerization of a-pinene oxide on HNT-Pir proceeded at a high rate and with an increase in the temperature of the reaction mixture (on 10-15 °C), which caused almost complete conversion of the substrate within 15 min of the reaction (Table 3).The main products of the reaction are campholenic and isocampholenic aldehydes, which in the case of cyclohexane are formed in a total amount of 57.8%.Similar selectivity values were observed in the same solvent on pretreated by HCl illite clay with comparable acidity (47.0 mmol g −1 ). 21n the presence of polar solvents containing an ester group (isopropyl acetate, dimethyl and diethyl carbonates, ethyl lactate), the main reaction products were campholenic aldehyde 2 (up to 51.2%) and trans-sorberol 7 (up to 28.0%), while the content of isomeric aldehyde 3 was insignicant (Table 3).In these media, the initial reaction rate (r 0 ) is lower than in hydrocarbons, however, in ethyl lactate, isomerization is completed within 1 h of the reaction.
In turn, a highly polar green solvent g-valerolactone (dielectric constant 3 36.4(ref.44)) causes the reaction to proceed at a relatively high initial rate with the preferential formation of trans-sorberol (45.8%,Table 3).According to ref.
26, the formation of this compound (up to 71%) was observed in the presence of polar weakly basic solvents (acetone, acetonitrile).
trans-Carveol 6 predominates in the reaction mixture (43.4%,Table 3) when using 2-methyl-tetrahydrofuran, which can be an environmentally friendly replacement for tetrahydrofuran, in the presence of which a-pinene oxide isomerizes on MoO 3 /beta to form almost equal amounts of campholenic aldehyde and the product 6 (ca.30%). 45hen using highly polar basic solvents (DMSO and DMF), higher temperatures were required for epoxide isomerization (90 and 140 °C, respectively), while almost complete substrate conversion was observed when DMSO was used.Selectivity to trans-carveol in these media was approximately the same, being 55-57% (Table 3).
To quantify the effect of solvents on selectivity, a comparison was made between the dielectric constant of the medium (3) and the lumped selectivity to the resulting aldehydes 2 and 3 (Fig. 10a), as well as alcohols 6 and 7 (Fig. 10b).Thus, with increasing solvent polarity, a decrease in selectivity for products  The relatively high lumped yield of products 6 and 7 when using weakly polar (3 6.97) 2-methyltetrahydrofuran may be associated with its relatively high basicity. 45omparison of the catalytic properties of KHN-Pir and HNT-Pir was carried out in cyclohexane, ethyl lactate and DMSO, since a high substrate conversion was observed on halloysite in these solvents, and the reactions proceeded with different selectivity values.
Kaolin AKF-78, as well as KNT synthesized from it, did not have any catalytic activity in the isomerization of a-pinene oxide in cyclohexane.The KNT/HNT-Pir materials in this solvent showed similar selectivity, with isomeric campholenic aldehydes as the main products (Table 4).At the same time, the reaction rate on KNT-Pir was lower, and for 99% conversion of the substrate required 3 h (Table 4), which may be due to the lower nature and concentration of a.s. in KNT-Pir (Table 2).Similar results were previously observed on modied by HCl illite clay, where Lewis a.s.predominated at a high degree of dealumination (L/B up to 6.7), which caused a decrease in the reaction rate, but the selectivity did not change signicantly. 21 the case of ethyl lactate, the reaction on KNT-Pir proceeded at a signicantly lower rate, and the substrate conversion aer 3 h was only 24%, which contrasts with the halloysite activity, where the reaction was completed aer 1 h of the reaction.This may be due to catalyst deactivation of the catalyst, probably due to the interactions of the active Lewis sites in KNT-Pir with the hydroxyl group of the solvent.
When using DMSO as a solvent, isomerization of a-pinene oxide occurs efficiently at 90 °C on aluminosilicate nanotubes of both types (KNT/HNT-Pir), leading to formation of trans-carveol 6 with almost the same selectivity (55-57%).A similar yield of product 6 (53.6%) was observed in the presence of MoO 3 /beta catalyst in the same solvent. 45ote that when using toxic solvents such as N,N-dimethylformamide and N,N-dimethylacetamide, selectivity towards trans-carveol was 43-44% on MoO 3 /beta at 70 °C, 45 50.6% in the presence of ammonium phosphotungstate on activated carbon 46 and 42% on hierarchical beta zeolites at 140 °C. 30The KNT-Pir and HNT-Pir catalytic systems in low-toxicity solvent, DMSO, can be used for the efficient isomerization of a-pinene oxide to valuable trans-carveol in the context of green chemistry.
The acidic properties of aluminosilicate nanotubes, in particular, halloysite, are due to the presence of Brønsted (H + , acidic -OH groups, polarized water molecules) and Lewis (exchange and structural Al 3+ ions) species on their surface. 4,35,37g. 10 Correlation between the dielectric constant of the solvent and selectivity for aldehydes 2 and 3 (a) as well as alcohols 6 and 7 (b).The Brønsted character of the clay acidity prevails at drying temperatures up to 200 °C due to the presence of adsorbed water on the surface, and a temperature increase leads to predominance of the Lewis sites. 47Based on drying HNT/KNT-Pir at 150 °C before the reaction, the Brønsted a.s.should determine their catalytic activity, therefore the mechanism of the studied reaction (Fig. 11) is considered from the point of view of protons as the active site.
At the rst stage, protonation of the substrate occurs with the formation of the 1-A carbocation, further transformations of which occur in several directions.Thus, when the cyclobutane ring of the 1-A ion expands, intermediates 1-B and 1-C can be formed, which leads to the formation of campholenic 2 and isocampholenic 3 aldehydes, respectively.Such reactions preferably occur in nonpolar solvents on weakly acidic catalysts, 21 as in the present case.][24][25] Another direction of 1-A transformation is the opening of the cyclobutane ring to form the carbocation 1-D, which can give trans-carveol 6 and trans-sorberol 7 as products (Fig. 11).Relatively selective formation of compound 6 occurs in the presence of polar basic solvents on catalysts with Brønsted 26-29 or both types of acidity. 30In the present work, this product was also formed in the presence of solids containing both Lewis and Brønsted centers.Isomerization of a-pinene oxide is considered as a reaction in which formation of the main products (campholenic aldehyde, trans-carveol) proceeds in a parallel course. 21,30,48The kinetic and thermodynamic aspects of these transformations have been thoroughly studied in the literature. 29,30,48,49verall, this work is the rst example of the utilization of functionalized synthetic kaolinite nanotubes as a catalyst.Future work may include investigation of various methods for their modication, including graing of functional groups and testing of catalytic activity.

Conclusion
Kaolinite nanotubes (KNT) were obtained by sequentially treating commercial kaolin AKF-78 (Uzbekistan) with dimethyl sulfoxide, methanol, and a solution of cetyltrimethylammonium chloride.The materials were characterized by EDX, XRD, FTIR, N 2 adsorption-desorption methods.Aer synthesis, nanotubes contain organic impurities, which complicates their further functionalization and use as catalysts.This drawback was successfully eliminated by two-stage treatment of KNT with a piranha solution (H 2 SO 4 -H 2 O 2 ), where at the rst stage organic impurities are effectively removed, and at the second the parameters of the porous structure (159 m 2 g −1 , 0.23 cm 3 g −1 ) and acidity (49 mmol g −1 ) of the nanotubes increased.The achieved parameters were close to those for halloysite nanotubes (HNT) treated with the same reagent.Both kaolin and halloysite nanotubes treated with H 2 SO 4 -H 2 O 2 show catalytic activity in the isomerization of a-pinene epoxide model reaction in various solvents, including so-called green ones.In non-polar media (cyclohexane) the formation of mainly campholenic aldehydes occurred, while polar basic solvents (DMSO, DMA) gave trans-carveol as the main product (55-57%).In general, treating KNT with a piranha solution is a simple and effective way to purify and modify them, allowing to increase porosity and acidity, which opens the way for further functionalization and practical use of these nanomaterials.

Fig. 3 X
Fig. 3 X-ray diffraction patterns of the studied kaolin from different deposits.

Fig. 4
Fig. 4 SEM images of the material obtained by sequential treatment of kaolin from Sulton Uvays (a) and Alyans (b) mines with DMSO, methanol and CTACl (0.1 mol L −1 ).

Table 1
Chemical composition and porous structure of the investigated solids

Table 2
Acidic properties of the investigated solids

Table 3
Initial screening of catalytic activity of HNT-Pir a in a-pinene oxide isomerization for 180 min in different solvents